Detinning



Patented May 14, 1940 UNITED STATES PATENT OFF-ICE nn'rmnmc.

scrap, tin-coated metals, tin

Anton Vollmer,

Berlin-Tegel, Germany, assignor to Metal & Thermit Corporation,

Carteret, N. 1.,

a corporation of New Jersey No Drawing. Ap

18 Claims.

to detinning; and it recovering tin from tin plate waste and the like, wherein such a material is immersed in a stripping bath comprising an aqueous alkaline solution of a nitro-substituted compound of the benzene series which toward anodic oxygen, I advantageously rebovered containing solution is stable and the dissolved tin is toward alkali and from the resulting tinby electrolytic methods; the

nitro compound which is reduced during the v stripping operation being anodic oxidation; all as set forth and as claimed.

usually regenerated by more fully hereinafter The present application is a continuation-inpart of my copending application,

Serial Number 81,127, filed May 21, 1936.

Several different methods of recovering t n from tin scrap have been proposed. One method which has been employed the use of an alkaline to some extent involves stripping bath in which an oxidizing agent is dissolved. Several different oxidizing agents have been proposed for this purpose. In one of these processes,

dissolving nitrous gases in a forming a mixture of nibath is prepared by caustic alkali solution, trites and nitrates.

nitrites and nitrates monia which escapes are Upon

partly reduced to am-. from the bath and is usufor example, the

solution of the tin the an ally lost. The tin is recovered by electrolysis of the solution, after which passed through the oxidizing agent.

nitrous gases must be bath in order to replace the I have found that any of the nitro substituted tageously, are be employed as oxidizing ping bath benzene compounds, which are soluble, as well as stable, in'alkaline solution and which, advanstable toward anodic oxygen, can

agents in a tin stripwith the production of wholly unexpected results. It has been found that a rate of stripping can .be obtained baths which may be rate obtained with prior it has been found possible compounds after the stripping operation in a very manner. K 3*} which are-particularly rep esena ed by thefollowing general formulaz N0 -Rr x,

p e l ,p

simple and effective The nitro compounds useful in the present wherein- R represents a invention can in these new stripping from 400 to 500 times the art stripping baths. And

to regenerate the nitro benzene,

naphthalene nucleus, which may be further substituted, while x represents a wat'er-' soli-ibilizing group such as -COOH, CH0,'-.C0N'H2,,etc.' g

in the molecule are alkyl,

which present O Na-, ther substituent groups plication April 1, 1937, Serial In Germany May 23, 1935 nitro, amino, halogen, alkoxy and additional water solubilizing groups. Specific compounds found useful include orthoand meta-nitrobenzoates, sodium ortho-nitrophenate, nitro-aminoephenol (1, 2, 4), nitro-amino-phenolsulfonic acid (1, 2, 6, 4) ortho-nitrobenzaldehyde, ortho-nitrobenzamide, ortho-nitrobenzene sulfonic acid, nitro-n'aphthol, and nitro-chloro-benzene sulfonic acid' (1, 6, 3). The nitro group in these compounds appears to be the active tin-stripping agent, the remainder of the molecule being of less importance. I have'found that many of the nitro compounds which form active tin-stripping agents can also be regenerated by anodic oxygen. These' regeneratable compounds are those which, dur- H ing solution of the tin, are reduced through the corresponding hydrazo compounds. I consequently prefer to use compounds of this nature, The above nitro compounds are useful as tinstripping agents over a wide range of ,concen trations. If desired the stripping operation ican be conducted by the addition of a nitro compound at intervals, starting with a-low concentration, or the bulk of the nitro compound which 2 is required may be added at'the start of the 1 process. The concentration of the caustic alkali in the stripping bath-is likewise subject to wide variation, although it' is advantageous to keep the concentration ofialkali'low fin -o er to prevent precipitation of stannatsinthe am. The I operative range of alkali-'isirom per cent by weight. 1

temperatures, however, s01 usually heat. the bath V to temperatures ranging from about 60 to C. Temperatures up to the boiling point of the bath can be employed. .Various methods can be employed for the regeneration of the nitro compound which is reduced during the stripping process.- If a compound is employed which is reduced through the corresponding hydrazo-compound, it is possible, as mentioned above, to, oxidize the latter to the originalnitrocompound by anodicoXyg'enQ The anodic oxygen which is generated during the electrolytic recovery of the tin may be employed for ithispurpose or another source may be used. I have found that the stripping process and the .[electrolytic recovery of the tin can be conducted in one and the same bath. The oxygen generated at the anode then automatically regenerates the simple and advantageous continuous process. Agitation of the bath with air or an oxygen-containing gas assists in the oxidation of the nitro compound. The air or oxygen may-be bubbled around the anode which is usually of iron or other insoluble metal. Agitation of the bath also speeds up the stripping action.

A large anode surface is advantageous and I have found it convenient to use a metal tank or vessel to hold the stripping bath and to use this tank as the anode. This tank may be made of iron, for example, having a thickness of 3 to 5 mm. When the stripping bath is employed in this manner both as a stripping bath and as an electrolytic cell, a stable condition is reached wherein the nitro compound is regenerated as fast as it is reduced. The electrolyte thus has an almost unlimited life. The tin content of the bath can be built up until it approaches the saturation limit of about 4 per cent by weight of tin.

If tin plate is being detinned the iron beneath the tin is not attacked by the stripping bath. The stripped metal can be rinsed with water and the wash water returned to the bath to make up for evaporation losses, for example.

It is possible, of course, to conduct the regeneration of the nitro compound by other methods. The stripping bath may be passed continuously through the anode compartment of a diaphragm cell, for example, in order to oxidize the hydrazo compounds which are formed, and then returned to the bath. It is also possible to extract the reduction products from the stripping bath with a water-insoluble organic extracting agent, such as benzene, to oxidize these compounds and then to return the resulting nitro compounds to the bath. When a diaphragm cell is used in the process, the tin scrap or other materialwhich is to be stripped can be placed in the anode compartment. The diaphragm prevents the oxidizing agent from reaching the cathode which tends to minimize current losses.

In order to illustrate the extreme rapidity with which tin can be stripped by means of' baths within the present invention, it is only necessary to cite the following I results which were obtained in a stripping bath containing 50 parts of meta-nitrobenzoic acid and 75 parts of caustic soda to 1000 parts of solution, the bath being maintained at a temperature of C. This bath was found capable of stripping 10 grams of tin, in the absence of agitation, and 12 grams when stirred, within a period of 15 minutes, with a total exposed tin surface of 300 square centimeters and with a bath volume of 400 cc. A similar bath containing nitrites and nitrates, produced by passing nitrous gasses through the bath, in substantially equivalent concentrations, and with other conditions the same, was found to dissolve only from 0.5 to 0.8

gram of tin within a period of 7 hours.

' stripping operation first to the azoxy compound,

then to the azo compound and finally to the hydrazo compound. It is possible that the azoxy compound and/or the azo compound may dissolve the tin in the stannous form. Whatever may be the exact course of the reduction, it is evident that the reduction products which are formed in my process may be readily re-oxidized to active tin stripping agents. The oxygen of the air appears to contribute to this oxidation.

7 through the bath at a caustic soda owing to its low cost.

results its concentration is maintained aboverate compartments for I compartments can be separated by means of a The tinwhich is deposited by electrolyzing the bath is glossy and homogeneous. The deposit is free from the dendritic type of deposit generally obtained from solutions containing stannous tin. Coatings having a thickness of even 15 mm. are found to be crystalline and homogeneous. -The material to be detinned may be placed in one of the usual rotary plating drums, which is rotated slowly during the stripping operation. This enables all tin-coated surfaces to be exposed to the stripping bath, while the rotation of the drum and the tumbling of the work contribute to the agitation of the bath.

It has been found advantageous to employ bipolar electrodes, in the combined stripping and electrolytic bath of the present invention. This assists in the oxidation of the reduction products in the bath and produces a better yield. A somewhat similar effect appears to be produced if the cathode is surrounded by means of a wire screen. A screen of iron provided with openings of 1 mm. has been found satisfactory. It is also possible to separate the cathode from. the anode by means of a porous plate, with beneficial results. The tin deposit formed on the cathode can be melted ofl whenever it has built up to a sufiicient thickness.

In one specific operation, which represents a practical embodiment of the present invention, I made up of an aqueous stripping bath containing 50 parts of metanitrobenzoic acid and parts of caustic soda to 1500 parts of solution. This bath was placed in an iron tank and heated to a temperature of 75 C. Scrap tin was then suspended in the bath and agitation was provided. The tin was rapidly stripped from the scrap and, as soon as the tin content was built up appreciably, a brass cathode was inserted in the bath, this cathode being surrounded on all sides by means of a wire screen having 1 mm. openings. The iron tank itself was connected as anode in an electric circuit and a current was passed voltage of about 2.5 volts, the current density being maintained below 3 amperes per square decimeter of cathode surface. As soon as the tin scrap was sufficiently detinned, additional scrap was added to the bath, the detinned scrap being removed, washed and the washings returned to the bath to make up for evaporation losses. This procedure was continued for 6% days. An over-all current efficiency of slightly over per cent was obtained in the electrolytic deposition. In other operations runs as long as 70 days with substantially the same results.

While I have described what I consider to be the best embodiments, various modifications can be made in the procedures described without'departing from the purview of my invention. Any of the caustic alkalis can be employed in making up the stripping bath but I prefer to employ For optimum about 4 per cent and not substantially exceeding about 9 per cent by weight. 0

One advantageous method of conducting the present invention is to employ a combined stripping and electrolytic bath but to provide sepathese operations. These porous diaphragm, or the surface of the bath can be separatedby a metal partition. In the latter case the surface of the electrolyte in the compartment in which the electrolysis is concompound of the benzene series, which is stable ducted can be covered with a thin layer of paraffln to prevent loss of heat and evaporation. The tin scrap may be continuously introduced and withdrawn from the stripping compartment by the use of an endless belt or other conventional means.

It is possible, of course, to recover metallic tin from my stripping bath by conventional methods other than those involving electrolytic deposition. It is equally possible to oxidize the reduction products derived from the nitro compounds by methods other than those specifically described.

The tin can be deposited from my stripping bath with a current efliciency ranging from about 90 to 99 per cent provided that the bath is kept substantially saturated with tin. Voltages of from 2.5 to 3 volts have been found satisfactory with current densities ranging up to 300 amperes per square meter of cathode surface (0.19 amps/sq. in.).

Other modifications of my process will be immediately evident to those skilled in the art. Such modifications as fall within the scope of the following claims I consider to be part of my present invention.

What I claim is:

1. In the process of detinning tin scrap, tin coated metals, tin plate waste and the like, the step which comprises immersing such a material in an aqueous bath containing a caustic alkali and a nitro-substituted organic compounds of the benzene .series dissolved therein under conditions producing stripping of the tin, said organic compound being stable towards saidalkali.

2. In the process of detinning tin scrap, tin

coated metals, tin plate waste and the like, the steps which comprise immersing such a material in an aqueous caustic solution containing dissolved therein a nitro-substituted organic compound of the benzene series under conditions producing solution of tin and simultaneous reduction of the nitro compound, and oxidizing the reduction products of said nitro compound; said nitro-substituted organic compound being one represented by the general formula NO2RX, wherein R represents a benzene, .diphenyl or naphthalene nucleus which may be further substituted while X represents one of the groups COOH, OH, ONa, SOaH, CONI-Is.

. 3. The process of claim 2 wherein said nitro compound is one capable of being reduced to the corresponding hydrazo compound during solution of said tin.

4. The process of claim 2 wherein the reduction products of said nitro compound are oxidized by anodic oxy en.

5. In the detinning of tin scrap, tin coated metals, tinplate waste and the like, the process toward anodic oxygen and said alkali, and'subjecting the resulting bath to the action of anodic oxygen, thereby oxidizing the reduction products of said nitro compound.

7. In the process of recovering tin from tin scrap, tin coated metals, tin plate waste and the like, the step which comprises immersing such a material in an aqueous solution of a caustic alkali containing dissolved therein an organic compound represented by the general formula NO2RX, wherein R represents a benzene, diphenyl, or naphthalene nucleus and Xrepresents a substituent group selected from a class consisting of --COOH, OH, '--ONa, SOsH, CH0 and -CONH2.

8. The process of claim 7 wherein the organic compound is an alkali metal meta-nitrobenzoate.

9. The process of claim 7 wherein the organic compound is an alkali metal ortho-nitrobenzo'ate.

10. The process of claim 7 wherein the organic compound is an alkalimetal ortho-nitrophenate.

dissolved tin electrolytically in the same bath during the stripping operation.

12. The process of claim 11 wherein the is connected as'anode in an electrical circuit.

13. The process of claim 11 wherein the bath is maintained at a temperature ranging from about C. up to the boiling point of the bath.

14. The process of claim 11 wherein the electrolysis is conducted with bipolar electrodes.

15. The process of claim 11 wherein the cathode is surrounded by a metallic screen.

16. The process of claim 11 wherein the electrolysis is conducted at a voltage of from about 2.5 to 3 volts and with a current density at the cathode not exceeding about 300 amperes per square meter.

17. The process which comprises stripping tin from scrap tin, tin coated metals, tin plate waste and the like by immersing such a material in an aqueous stripping bath containing sodium meta-nitrobenzoate and caustic soda of a concentration ranging from about 4 to 9 per cent by weight, continuing the stripping action until the bath becomes substantially saturated with tin and electrolytically depositing the dissolved tin in the same bath.

18. In the recovery of tin from scrap tin, tin coated metals, tin plate waste and other tin-bearstripping bath is'contained in a metal tank which ing materials, the process which comprises establishing and maintaining a bath having a stripping compartment and an electrolyzing compartment; said bath containing an aqueous solution of caustic soda and a dissolved nitro-substituted organic compound of the benzene series, stable in alkaline solution and toward anodic oxygen, and being maintained at a temperature ranging from about 60 C. up tothe boiling point of the bath; continuously passing tin-bearing materials through the stripping compartment of said bath and continuously depositing tin electrolytically in the electrolytic compartment of said bath.

mom vouama. 

